Aluminum oxynitride is a ceramic material that has been proposed for use in the fabrication of transparent articles such as vehicle windshields, and vehicle and building windows. Aluminum oxynitride offers the potential of a high degree of ballistic protection with a commensurate weight savings compared to traditional glass/polymer-based transparent armor. Transparent aluminum oxynitride has been shown to defeat lethal armor-piercing rounds at a fraction of the thickness and overall weight of glass/polymer laminates. However, aluminum oxynitride has not found widespread application due to the difficulty and expense of fabricating large panels for such applications as windshield and windows.
A typical present prior art process of fabricating transparent aluminum oxynitride starts with a homogeneous mixture of aluminum oxide and aluminum nitride powders. Such is mechanically milled and heated, and then mechanically sieved to form a desired aluminum oxynitride powder which, at this point, is very low in percentage of maximum theoretical density. The sieved powder is typically mixed with lubricants and binders, and then typically formed using a die and pressing apparatus into some suitable self-sustaining mass typically referred to as a green part or cold compact. The cold compact might be subsequently subjected to a hot pressing operation to provide a near-final shape of the aluminum oxynitride object that is desired. Such typically results from mechanically pressing the cold compact within a suitable die, typically at a temperature of from 800° C. to 2,000° C. The cold compact is typically at about 65% of maximum theoretical density, whereas at the conclusion of hot pressing a minimum of approximately 92% of maximum theoretical density is achieved. Consider that the hot press-final shape is somewhat limited by the nature of the mechanical pressing into a desired mold/die, and is typically conducted to create simple, very thin, substantially planar, and/or perhaps conical shaped products.
To achieve desired finished optical and ballistic product properties, 92% maximum theoretical density is typically insufficient. Therefore, the hot pressed object is subjected to hot isostatic pressing. Alternately, a cold compact without hot die/mold pressing might be directly subjected to hot isostatic pressing. Such essentially is a process whereby the object is provided into a pressure vessel that is heated to at least 1,800° C. and pressurized to an ambient internal chamber pressure typically at 20,000 psig and greater for a suitable period of time to achieve 99% or greater of maximum theoretical density. There is considerable cost and risk associated with such high pressure vessels, and the engineering and cost considerations become prohibitive for the creation of large aluminum oxynitride objects.
While the invention was motivated in addressing the above identified issues, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded, without interpretive or other limiting reference to the specification, and in accordance with the doctrine of equivalents.